1. Field of the Invention
This invention relates to a developer for pressure-sensitive recording sheet, its aqueous dispersion, and a method for preparing the developer.
2. Description of Related Art
As described in U.S. Pat. Nos. 2,712,507, 2,800,457 and 3,418,250, pressure-sensitive recording sheets make use of combinations of electron donative colorless dye (hereinafter referred to simply as "dye") solutions encapsulated in microcapsules and electron acceptive compounds (hereinafter referred to simply as "developer"). When the microcapsules are broken by application of a pressure, the dye solution and the developer are contacted with each other and reacted to develop a color. In general, this type of sheet is called no carbon required paper and has widely been used.
The currently employed developers for the pressure-sensitive recording sheets include, aside from inorganic compounds such as activated clay, organic compounds such as formaldehyde polycondensates of para-substituted phenols, metal salts of nuclear substituted salicylic acids (Japanese Laid-open Patent Application Nos. 51-25174, 62-19486, 62-176875, 62-178387 and 62-178388). The present invention is concerned with metals salts of the nuclear substituted salicylic acids.
Most of polyvalent metal salts of nuclear substituted salicylic acids are insoluble in water and are superior as a developer to other developers with respect to the high color density and the stability in color image, thus being widely utilized. In general, these metal salts are resinous materials having high softening points. In most cases, they are divided into fine pieces and dispersed in water by means of pulverizers or mills such as a ball mill, an attritor, a sand grinder and the like. Subsequently, inorganic pigments, clay minerals, adhesives, dispersants and defoamers are added to the dispersion thereby preparing a coating composition, followed by coating and deposition on a substrate. Therefore, amorphous compounds of the polyvalent metal salts of nuclear substituted salicylic acids which have a relatively high softening point suitable for the division into fine pieces and dispersion have been in use as a developer. However, the developer having a high softening point has the following two disadvantages.
(1) Organic developers having a high softening point are so slow in rates of dissolution in a dye solution encapsulated in microcapsules, reaction with the dye and color development that it takes a long time before the density or color tone of a color image becomes constant after the microcapsules have been broken to permit the dye solution and the developer to contact. This tendency becomes more pronounced at lower temperatures, and a gentle variation of the color density with time has the relation with an instantaneous color developing characteristic. The variation of the color tone with time is considered as a problem especially for the development of black color from a plurality of mixed dyes.
(2) Developers having a high softening or melting point do not adhere to a substrate by softening or melting at a drying temperature of coating composition and thus require a slightly larger amount of an adhesive for fixing on the substrate. This tends to become more pronounced when using finer developers particles in order to improve the color density or instantaneous color developing properties, with the great possibility that one is disenabled to attain the purpose by the action of impeding the color development of the adhesive.
The softening point of the polyvalent metal salts of nuclear substituted salicylic acids is in close relation with the structure of the substituent. If a ring structure is contained as the substituent, the softening point is generally high. However, polyvalent metal salts of nuclear substituted salicylic acids having low softening and melting points are not always obtained solely based on the fact that any ring structure is not contained as a substituent. Polyvalents metal salts of nuclear substituted salicylic acids which are free of any ring structure as a substituent have been hitherto proposed including anacardic acid, 5-tertiary octylsalicylic acid, 3-methyl-5-tertiary octylsalicylic acid, 3,5-di-tertiary butylsalicylic acid, 3,5-di-tertiary amylsalicylic acid, 3-tertiary octyl-5-methylsalicylic acid or 3-tertiary octyl-5-ethylsalicylic acid. However, these are all compounds having a high melting point and are sparingly soluble in dye solution, so that a satisfactorily high color density cannot be attained.